Aromatic carboxylic acids, such as terephthalic acid, isophthalic acid, and napthlene dicarboxylic acid are useful chemical compounds and are raw materials in the production of polyesters and copolyesters. In the instance of terephthalic acid, a single manufacturing facility can produce greater than 100,000 metric tons per annum as feedstock for a polyethylene terephthalate (PET) facility.
Terephthalic acid (TPA) can be produced by the high pressure, exothermic oxidation of a suitable aromatic feedstock such as para-xylene in a solvent such as acetic acid, water, or mixtures thereof. Typically, these oxidations are carried out in a liquid phase using air or alternate sources of molecular oxygen in the presence of metal catalyst(s) or promoter compound(s). Methods for oxidizing para-xylene and other aromatic compounds such as m-xylene and dimethylnapthlene are well known in the art.
In addition to oxidation, many industrial TPA processes also incorporate a hydrotreating (hydrogenation) process to produce a so-called purified terephthalic acid or PTA. Typically, these processes are conducted using a water solvent. These hydrogenation processes are also well known in the art.
TPA processes give rise to high temperature material streams. These streams are derived from both the heating needs within the process and heat removal needs. Typical would be to provide some sort of heat input which would result in the boiling or evaporation of a solvent. Energy could then be recovered via condensation of the solvent.
As the art has progressed, a significant body of literature has been developed to address the problem of efficient energy recovery within the TPA process. In general, these schemes usually involve the recovery of useful work/electricity via the use of a turbine and/or the recovery of heat energy via the use of steam generation. Both of these general schemes have certain drawbacks and limitations.
In the case of recovery of work/electricity via the use of a turbine, there are significant technical and economic problems. In the case of steam generation, the technical problems of physically producing steam are relatively insignificant. However, the limitations are generally associated with the usefulness of the steam generated. Specifically, the steam generated from TPA processes generally is at too low a temperature and/or pressure to be useful as a heating medium in general. And specifically within the remainder of the process.
Although, by themselves mechanical energy recovery via a turbine and heat energy recovery via steam generation are not necessarily novel, the objective of this invention is to describe a method of heat energy recovery involving the generation of steam followed by processing of the generated steam into a more useful form.